dexamethasone suppression test

Indications

• diagnosis of Cushing's syndrome

Reference interval

• serum cortisol < 1.8-3 ug/dL (< 5 ug/dL); > 1.8 ug/dL (MKSAP20)

Principle

• Dexamethasone suppresses pituitary secretion of ACTH which, in turn, suppresses endogenous secretion of cortisol.
• cortisol assays do NOT detect dexamethasone

Predictive value of a positive test is poor (0.6%)

Predictive value of a negative test is close to 100%.

Procedure

• • dexamethasone: 1 mg at 11 PM
  • serum cortisol at 8 AM

Interferences

• False positives occur with:
• physiologic stress, psychologic stress
• renal failure
• estrogen therapy
• pharmaceutical agents
  • phenytoin
  • phenobarbital
  • carbamazepine
  • rifampin
• endogenous depression
• incorrect timing of dexamethasone administration or of blood sampling
• significant obesity
• alcoholism
• anorexia nervosa

Management

• if 8 AM serum cortisol > 5 ug/dL (> 1.8 ug/dL MKSAP20)
• serum cortisol < 1.8-3 ug/dL (< 5 ug/dL); > 1.8 ug/dL MKSAP20)
  • serum ACTH*
  • serum DHEA-sulfate
  • urine free cortisol
  • 8 mg dexamethasone suppression test* if 1 mg of dexamethasone does not fully suppress serum cortisol
    • serum cortisol suppresses with ACTH-secreting pituitary adenoma*
    • serum cortisol does not suppress with ectopic ACTH-secreting tumor

* serum ACTH before 8 mg dexamethasone suppression test or inferior petrosal sinus ACTH

* intrapetrosal sinus sampling identifying an ACTH-secreting pituitary adenoma renders an 8 mg dexamethasone suppression test unnecessary

More general terms

• provocative clinical test

Additional terms

• cortisol in serum
• dexamethasone (Decadron, Maxidex, AK-Dex, Ozurdex)